The present invention relates to control systems used for heating substances and in particular to control systems for temperature process controllers for controlling the rate at which power is delivered to heating elements for heating substances, and their method of operation.
In modern laboratory analysis, accurate control of reaction pathways or processes requires that measurements be made at a controlled temperature. Process controllers, which are commercially available, are used to control the heating of a heating element, such as a heating mantle, which heats a substance in a flask on the mantle. A typical process controller includes a control panel for entering a set point temperature which represents the temperature to which the substance is to be heated. A temperature sensing probe is inserted into the substance and connected to the process controller for providing a sensed temperature to the process controller. The process controller compares the set point temperature to the sensed temperature. When the set point temperature is greater than the sensed temperature, the process controller provides an output signal that either (1) directly powers the heating mantle, or (2) powers an intermediate device that powers the heating mantle. In one configuration of the above, the presence of the output signal closes the contacts of the solid state relay to provide power, such 120 VAC, to operate the heating element. As the sensed temperature begins to approach the set point temperature, the controller stops providing the signal on a continuous basis to the relay and the output signal is provided on a proportional basis to the relay. As the difference between the sensed temperature and the set point temperature continues to narrow, the ratio of the on time/off time of the output signal decreases until a ratio is established that maintains the sensed temperature at the set point temperature.
Commerically available process controllers are acceptable in situations when the heater power at 120 VAC is appropriately matched to the mass being heated. However, in a small scale research environment, the heating power of the common heating element may be many times greater than that required for the mass being heated. When a process controller is used in this situation (i.e., high heater efficiency, low mass) it results in the process controller significantly exceeding the set point temperature on initial warm-up and sporadic temperature control thereafter. Thus, controlling this overshoot of the set point temperature is critically important. In the past, power supplied to the heating mantle has been reduced by placing a variable resistor such as a variac between the power source and the mantle. However, this approach has proved to be inaccurate and difficult to control.